Sinclair ZX80 / ZX81 / Z88 Forums

Hi,

Anybody know why the TS2068 needs 15v in instead of the 9v of everything else (except the QL)? We are working on a universal power supply to get rid of the rats nest of walwarts, and the TS2068 complicates things.

Because it uses a 12V series pass voltage regulator. This needs its input to be 15V DC nominal.

Mark

Thanks. Is there an ignorant person translation? So, instead of a typical 7805 going from 9v to 5v, this is going from 15V to 12V? If so, what's the different requirement that it needs 12v? Sorry, I've only the minimal understanding of circuits.

Thanks.

Yes: the 78L12 needs 15 volts to deliver 12. The 12v is mainly for the video chip. Even so, the it’s under sized. Replacing the 78L12 with a 7812 will improve the video a bit.

The chip that performs colour encoding and modulation of the video signal (for the colour composite video and the RF outputs) requires a stable, regulated +12V DC supply.

In the Sinclair ZX Spectrum, a step up DC-DC converter was used to convert the +9V DC input to +12V for the chips that form the ‘lower’ RAM (where the screen memory display is located) and for the chip that performs colour encoding.

A separate circuit based on a 7805 series pass voltage regulator chip converts the +9V supply to +5V, which then supplies all the digital logic chips, such as the ULA, Z80, ROM, ‘upper’ RAM etc.

In the Timex TS2068 they are using completely different power circuitry. Instead they are using a 15V external supply, then using a 78L12 series pass voltage regulator chip to convert the +15V supply to +12V. This then feeds the chip that performs colour encoding and modulation of the video signal.

A separate DC-DC converter circuit converts the +15V to +5V for the all the digital logic chips (Z80, ROM, RAM etc.).

All the 78xx series pass voltage regulator chips (like the 7805, 78L12, 7812) need at least 2.5V headroom on the input terminal above the output voltage that they are designed to provide. It’s a bit like a gravity fed water system. The water tank has to be higher than the water tap/faucet, plus some extra height so that there is enough pressure for a reasonable water flow.

Mark

Awesome,

Thanks for the education